I've generally been pretty happy with T-mobile. Speaking broadly, I can say that their coverage is not as good as AT&T or Verizon, especially when I get far from the city. However, in my case, where 98% of my time is spent within 10 miles of Boston, I'm very happy to not be paying the rates charged by either of the former. When I added my fiance's line to my T-mobile account, I did have some issues that took a bit of effort to resolve, largely the result of the in-person sales associate being a bit of a doofus from what I could tell.
Both our phones were brought in ourself, and are not carrier locked. Going forward, I don't know that I'd ever get a phone directly through a provider again, as I like the flexibility to switch if needed. We have 1000 minutes, unlimited texts, and 2GB of data each (data still works beyond that, is just slower), for $90/month. For us, it's a good deal.